1. Field of the Invention
The present invention relates to a liquid crystal display device used for displaying characters and images, and a liquid crystal display apparatus incorporating the same.
2. Description of the Background Art
Liquid crystal display apparatuses, being thin and light in weight, are used in a wide variety of applications such as displays for personal digital assistance devices. Such a liquid crystal display apparatus includes a panel-shaped liquid crystal display device and other components.
A passive matrix type liquid crystal display device will now be described with reference to FIG. 10, as an example of a liquid crystal display device.
FIG. 10 schematically illustrates a cross section of a liquid crystal display device 101.
The liquid crystal display device 101 includes a first substrate 102, a second substrate 103 opposing the first substrate 102, and a liquid crystal layer 106 provided between the substrates 102 and 103 that are sealed together along the periphery thereof by a sealant 104 containing spherical beads 104. The liquid crystal display device 101 has a display area in which characters and shapes are displayed, and a non-display area surrounding the display area. In the display area, a plurality of strip-shaped first display electrodes 109a are provided on one side of the first substrate 102 that is closer to the liquid crystal layer 106, and a plurality of strip-shaped second display electrodes 111a are provided on one side of the second substrate 103 that is closer to the liquid crystal layer 106. The first display electrodes 109a extend in a direction vertical to the sheet of the figure and are spaced apart from one another. The second display electrodes 111a extend in the left-right direction in the figure and are spaced apart from one another. A rectangular pixel is defined by these electrodes 109a and 111a at each intersection therebetween, whereby a plurality of such pixels are arranged in a lattice-shaped matrix pattern. In the non-display area, first peripheral electrodes 109b are provided with substantially no interval therebetween on one side of the first substrate 102 that is closer to the liquid crystal layer 106 so as to surround the first display electrodes 109a. The first peripheral electrodes 109b include, for example, wiring electrodes that are connected to the first display electrodes 109a, and dummy electrodes that are provided so that the non-display area is occupied by electrodes as is the display area. First electrodes 109 include the first peripheral electrodes 109b and the first display electrodes 109a in the display area. A first alignment film 110 and a second alignment film 112 for regulating the orientation direction of the liquid crystal molecules are provided respectively on one side of the first display electrodes 109a that is closer to the liquid crystal layer 106 and on one side of the second display electrodes 111a that is closer to the liquid crystal layer 106. For each pixel of the liquid crystal display device 101, the voltage to be applied across the liquid crystal layer 106 between one first display electrode 109a and one second display electrode 111a that define the pixel is controlled so as to change the orientation of the liquid crystal molecules in the liquid crystal layer 106 and to adjust the amount of light to be transmitted therethrough, thus displaying characters and images in the display area.
The display quality of the liquid crystal display device 101 is substantially influenced by variations in the thickness of the liquid crystal layer 106, i.e., variations in the cell gap. As the variations increase, the display quality decreases, causing a display non-uniformity, or the like. Therefore, in order to achieve a good display quality, it is necessary to achieve a uniform cell gap.
In order to address the problem, a large number of spherical spacers are provided between the first and second substrates in the prior art. However, such spherical spacers are easily collapsed by an external force because they support the substrates by point contact, and the particle diameter thereof is in conformity with the normal distribution, whereby it is not possible to achieve a highly uniform cell gap with such spherical spacers. Moreover, spherical spacers have small contact areas with the substrates because they support the substrates by point contact as described above. Therefore, even if the spherical spacers are fixed by providing a fixing layer or an attachment layer, they are moved in the liquid crystal layer by an external force such as a vibration or a shock applied thereto, thereby causing a change in the spacer distribution, which in turn influences the cell gap variations.
In view of this, a different type of liquid crystal display device has been proposed and put into practical use, in which a large number of columnar spacers are provided so as to be spaced apart from one another between the opposing surfaces of the first substrate and the second substrate.
For example, Japanese Laid-Open Patent Publication No. 6-222370 discloses a method for fixing columnar spacers along gaps in a transparent electrode by forming a transparent electrode pattern on a surface of a substrate by using a photoresist, baking the photoresist remaining on the pattern to convert it into a lift-off mask, forming a photosensitive film having a predetermined thickness on the surface of the substrate, exposing and developing the photosensitive film from the reverse surface of the substrate via the lift-off mask so as to form the columnar spacers that are aligned with the gaps in the transparent electrode pattern.
Moreover, Japanese Laid-Open Patent Publication No. 2000-298282 discloses a liquid crystal display apparatus including a pair of substrates with a liquid crystal layer being interposed therebetween, a first electrode provided on one of the substrates in a pixel region, a second electrode provided on the other substrate in a pixel region, and a columnar spacer provided on one of the substrates in the pixel region, the columnar spacer being covered by one of the first and second electrodes that is provided on the one of the substrates. Japanese Laid-Open Patent Publication No. 2000-298282 discloses that with such a configuration, the direction of an electric field produced between a portion of the electrode that is formed on the side surface of the columnar spacer and the other electrode is different from that of an electric field produced between another portion of the electrode and the other electrode, thereby providing a so-called xe2x80x9cmulti-domainxe2x80x9d effect in this area.
Such columnar spacers are formed by applying and pre-baking a material obtained by adding a photosensitizer to an acrylic resin liquid, or the like, on the first substrate on which an electrode pattern has been formed, subjecting the material to a UV exposure/development process using a photomask, and then baking the material.
The cell gap uniformity is required not only in the display area in which characters and images are displayed, but also in the non-display area surrounding the display area, because it substantially influences the display quality in the peripheral portion of the display area. Particularly, in an STN (Super Twisted Nematic) liquid crystal display device, in which an image is displayed by controlling not only the optical rotatory of the liquid crystal material but also the birefringence thereof, the retardation is sensitive to variations in the cell gap, thereby imposing very high requirements as to the uniformity of the cell gap in the display area and in the non-display area. Liquid crystal display devices using ferroelectric liquid crystal materials also impose such high requirements because of their very small cell gaps. In order to address such requirements, columnar spacers are usually provided not only in the display area but also in the non-display area.
However, the provision of columnar spacers has the following problems.
Typically, columnar spacers are provided in the display area in a pattern such that the columnar spacers are placed within the gaps between display electrodes on the first substrate in order to increase the pixel aperture ratio and thus to improve the optical characteristics such as the brightness and the contrast, and the columnar spacers are formed in the non-display area in the same pattern as in the display area for ease of the manufacturing process. However, in the non-display area, the peripheral electrodes such as the wiring electrodes and the dummy electrodes are provided in a complicated pattern with substantially no interval therebetween, whereby not a few of the columnar spacers are formed on the peripheral electrodes. As described above, when forming the columnar spacers, a resin liquid is applied on the first substrate on which the first electrodes have been formed. In this process, there occurs a difference of about 1500 to 2000 xc3x85 between the height of the top surface position of a resin layer 131 in a portion where the first electrode 109 is present and that in another portion where the first electrode 109 is absent, as illustrated in FIG. 11A (in FIG. 11A to FIG. 11C, the same reference numerals are used to denote the same elements as those in FIG. 10). Therefore, the position of the top surface of a columnar spacer 113 formed within a gap between the first display electrodes 109a in the display area is lower than that of another columnar spacer 113 formed on a peripheral electrode in the non-display area, as illustrated in FIG. 11B. As a result, the cell gap in the display area is different from that in the non-display area, as illustrated in FIG. 11C, thereby causing a display non-uniformity in the peripheral portion of the display area and thus decreasing the display quality.
Assumedly, this problem can be addressed by arranging each columnar spacer within a gap between the peripheral electrodes also in the non-display area. However, since the gap is much narrower than that in the display area, only small-sized columnar spacers can be provided, and such small-sized columnar spacers may possibly be damaged during an alignment film rubbing treatment. Moreover, it is difficult to realize such a process that is capable of forming columnar spacers precisely within such small gaps. In addition, the arrangement density, the size and the shape of the columnar spacers are restricted by the size and the application of the liquid crystal display device.
An object of the present invention is to provide a liquid crystal display device having a very good cell gap uniformity in a boundary region between a display area and a non-display area, and a liquid crystal display apparatus incorporating the same. In the present invention, columnar spacers that define the thickness of a liquid crystal layer, i.e., the cell gap, are provided on electrodes both in the display area and in the non-display area. Therefore, unlike in a case where columnar spacers are provided within inter-electrode gaps in the display area, there is substantially no difference in the height of the top surface position of the spacers between the display area and the non-display area, thus realizing a very good cell gap uniformity in the boundary region.
Specifically, a liquid crystal display device of the present invention includes: a first substrate; a second substrate opposing the first substrate via a large number of columnar spacers therebetween, the columnar spacers being fixed on the first substrate; and a liquid crystal layer provided between the first substrate and the second substrate, wherein: the liquid crystal display device has a display area for displaying an image, and a non-display area adjacent to the display area; in the display area, a plurality of display electrodes that are used when applying a voltage across the liquid crystal layer are provided at an interval on one side of the first substrate that is closer to the liquid crystal layer; in the non-display area, a plurality of peripheral electrodes are provided with substantially no interval therebetween on the side of the first substrate that is closer to the liquid crystal layer; and among the large number of columnar spacers, the columnar spacers in the display area are provided on the display electrodes while the columnar spacers that define a thickness of the liquid crystal layer in the non-display area are provided on the peripheral electrodes.
Herein, the peripheral electrodes provided on the first substrate in the non-display area include wiring electrodes that are connected to the display electrodes, and dummy electrodes that are provided so that the non-display area is occupied by electrodes as is the display area. All of the columnar spacers provided in the non-display area may be intentionally provided on the peripheral electrodes. Alternatively, the columnar spacers may be provided so that some of the columnar spacers are placed on the peripheral electrodes and thus define the cell gap, while the other columnar spacers are placed not on the peripheral electrodes but within inter-electrode gaps and thus are not involved in defining the cell gap, as in a case where the columnar spacers are provided in a predetermined pattern in the non-display area.
The cell gap is influenced by the substrate contact area and the arrangement density per unit area of the columnar spacers. For example, the cell gap tends to increase as the substrate contact area per unit area of the columnar spacers increases or as the arrangement density thereof increases. Thus, the cell gap may be varied by variations in the size of the columnar spacers or the arrangement density thereof Therefore, in the liquid crystal display device of the present invention, it is preferred that the large number of columnar spacers on the first substrate are all of the same size and are arranged in a uniform pattern across the entire area including the display area and the non-display area. In this way, it is not necessary to form, in separate processes, different types of columnar spacers in the display area and in the non-display area, and the columnar spacers of the same shape can be formed simultaneously in a uniform pattern across the entire area including the display area and the non-display area, thereby providing the following advantages: (a) the number of process steps for forming the columnar spacers is reduced, thus improving the productivity; (b) only one process is required for forming the columnar spacers, thereby suppressing the deterioration of the first substrate due to the process; and (c) the amount of material to be consumed for forming the columnar spacers is reduced, thereby reducing the cost. Herein, xe2x80x9call of the same sizexe2x80x9d means that the variations in the cross-sectional area (contact area with electrodes) of the columnar spacers are xc2x120% or less, and xe2x80x9cuniform patternxe2x80x9d means that the variations in the arrangement interval are xc2x120% or less. Thus, the size and the arrangement pattern of the columnar spacers can be designed so as to fall within such value ranges. The columnar spacers can be designed by blocks, i.e., separately for the display area and for the non-display area, in which case the design of the columnar spacers can be facilitated and standardized.
Typically, in the display area, all the columnar spacers are interposed between the display electrodes on the first substrate and the display electrodes on the second substrate (hereinafter expressed as xe2x80x9cinterposed between upper and lower electrodesxe2x80x9d). In the non-display area, however, the peripheral electrodes are not necessarily arranged regularly, and the peripheral electrodes may be extraction electrodes that are arranged in a bent pattern or dummy electrodes whose width is smaller than that of the display electrodes, whereby it is very difficult for all the columnar spacers in the non-display area to be interposed between the upper and lower electrodes. However, it can be inferred from the experiments described later (see TABLE 10) that as long as 40% or more of the columnar spacers in the non-display area are interposed between the peripheral electrodes on the first substrate and second peripheral electrodes on the second substrate, the cell gap can be effectively defined by those columnar spacers. Therefore, in the liquid crystal display device of the present invention, in a case where the columnar spacers are all of the same size and are arranged in a uniform pattern, it is preferred that: second peripheral electrodes are provided on one side of the second substrate that is closer to the liquid crystal layer; and 40% or more of the columnar spacers in the non-display area are interposed between the peripheral electrodes on the first substrate and the second peripheral electrodes on the second substrate. Note that where columnar spacers are provided so as to be placed within inter-electrode gaps in the display area, as in the prior art, the columnar spacers that are interposed between the upper and lower electrodes in the non-display area will protrude and thus disturb the cell gap. In contrast, in the present invention, the cell gap is defined by those columnar spacers that are interposed between the upper and lower electrodes, whereby even if there are columnar spacers that are not interposed between the upper and lower electrodes, they do not influence the cell gap.
If the minimum outside diameter of the columnar spacers is smaller than 6 xcexcm, the columnar spacers may be damaged during the alignment film rubbing treatment. If the minimum outside diameter is greater than 50 xcexcm, the alignment film rubbing treatment may be insufficient, particularly in an area around the base of each columnar spacer, by being hindered by the presence of the columnar spacers, thereby causing a display non-uniformity, roughness, etc., resulting in a poor display quality. Therefore, in the liquid crystal display device of the present invention, in a case where the columnar spacers are all of the same size and are arranged in a uniform pattern, it is preferred that the minimum outside diameter of the columnar spacers is 6 to 50 xcexcm. Herein, the minimum outside diameter of a columnar spacer is, for example, the diameter of the columnar spacer if the spacer cross section is circular, or the length of each side of the spacer cross section if the cross section is square-shaped.
In a case where the columnar spacers are arranged in a uniform pattern across the entire area including the display area and the non-display area, the columnar spacers can be placed on the peripheral electrodes with a high probability in areas where the inter-electrode interval is smaller than 15 xcexcm, whereby it is possible to achieve a uniform cell gap in the boundary region between the display area and the non-display area. Therefore, in the liquid crystal display device of the present invention, in a case where the columnar spacers are all of the same size and are arranged in a uniform pattern, it is preferred that the peripheral electrodes have areas where an inter-electrode interval therebetween is smaller than 15 xcexcm.
If the non-display area is provided so as to surround the display area, and the peripheral electrodes (extraction electrodes, dummy electrodes, etc.) are provided in the non-display area so as to surround the display electrodes in the display area, it is possible to design a liquid crystal display device having a uniform cell gap with a simple design by providing the columnar spacers of the same size and arranging them in a uniform pattern in the display area and the non-display area. Therefore, in the liquid crystal display device of the present invention, in a case where the columnar spacers are all of the same size and are arranged in a uniform pattern, it is preferred that the non-display area is provided so as to surround the display area, and the plurality of peripheral electrodes in the non-display area are provided so as to surround the display electrodes in the display area.
A substrate made of plastic expands and contracts due to a temperature change by a greater amount as compared to a substrate made of glass, whereby the positions of columnar spacers formed on a substrate can be easily shifted. Therefore, in a case where the columnar spacers are placed within the inter-electrode gaps in the display area, there is required a precisely-controlled process. However, in the present invention where the columnar spacers are placed on electrodes that occupy significantly greater areas, such a precisely-controlled process is not necessary for forming the columnar spacers. Thus, the liquid crystal display device of the present invention is suitable for cases where the first substrate is made of a plastic.
Display electrodes that are formed by a sputtering method, or the like, have minute irregularities present on the surface thereof, whereby the fixing of the columnar spacers on the electrodes is strengthened due to, for example, the increased contact area therebetween as compared to that in a case where the columnar spacers are fixed on the surface of a flat first substrate made of glass, or the like. Therefore, in the liquid crystal display device of the present invention, it is preferred that the columnar spacers in the display area are directly fixed on the display electrodes, and the columnar spacers in the non-display area are directly fixed on the peripheral electrodes.
An STN liquid crystal display device displays an image by controlling not only the optical rotatory of the liquid crystal material but also the birefringence thereof, whereby the retardation is sensitive to variations in the cell gap. Therefore, the liquid crystal display device of the present invention is particularly advantageous when the display mode is an STN mode.
The liquid crystal display device of the present invention, when incorporated together with other components, provides a liquid crystal display apparatus having a good display quality even in the peripheral portion of the display area.
Further objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings.